Conventionally, implementing an electronic shutter operation by executing a reset scan by pixel or by line to remove unneeded charges accumulated at the pixels and then carrying out a scan by pixel or by line to output the signal charge after a predetermined period of time has elapsed for each pixel or for each line is known.
A description is now given of an electronic shutter operation in an image sensing apparatus that uses an image sensing device that employs a conventional XY address-type scan method. Specifically, using FIG. 11 and FIG. 12, a description is given of the structure of the conventional image sensing device and the drive method called rolling electronic shutter operation among the electronic shutter operations.
FIG. 11 is a schematic diagram showing the structure of an image sensing device employing the conventional XY address-type scan method.
Reference numeral 101 designates a unit pixel, with multiple unit pixels 101 arranged in a matrix. Reference numeral 102 designates a photodiode (hereinafter “PD”) that converts light of an object image into a signal charge. Reference numeral 104 designates an area that temporarily stores the signal charge (that is, a floating diffusion part, hereinafter referred to as “FD”). Reference numeral 103 designates a transfer switch that transfers the signal charge generated at the PD 102 to the FD 104 using a transfer pulse φTX. Reference numeral 105 designates a MOS amplifier that functions as a source follower. Reference numeral 106 designates a selection switch that selects a unit pixel 101 using a selection pulse φSEL. Reference numeral 107 designates a reset switch that resets the FD 104 to a predetermined potential (VDD) using a reset pulse φRES. A floating diffusion amplifier is composed of the FD 104, the MOS amplifier 105 and a constant current source 109 that is described below. The signal charge of the unit pixel 101 selected by the selection switch 106 is converted into voltage and output to an output circuit 111 over a signal line 108. Reference numeral 109 designates the constant current source that becomes a load of the MOS amplifier 105.
By the driving of a horizontal scan circuit 110, the output from the pixels 101 are output from the output circuit 111 to an output line 112. In addition, reference numeral 113 designates a vertical scan circuit that controls the driving of the pixels by supplying the respective drive signals φTX, φSEL and φRES to the switches 103, 106 and 107, respectively. In φTX, φSEL and φRES, respectively, the drive signals supplied to an nth scan line selected for scanning by the vertical scan circuit 113 are written as φTXn, φSELn and φRESn.
FIG. 12 is a schematic diagram showing drive pulse and sequence of a rolling electronic shutter operation. It should be noted that FIG. 11 describes an example of the driving of from a line n to a line n+3 by the vertical scan circuit 113.
With the rolling electronic shutter operation, in line n, first, between a time t31 and a time t32, pulses are applied to φRESn and φTXn and the transfer switch 103 and the reset switch are turned on, removing the unneeded charges accumulated in PD 102 and FD 104 of each pixel on the line n and resetting them to a predetermined potential. At time t32, the transfer switch 103 is turned off and the light charge generated at the PD 102 begins to be accumulated. The charge generated at the PD 102 by photoelectric conversion is called “light charge”, hereinafter.
Next, at a time t34, a pulse is applied to φTXn and the transfer switch 103 is turned on, transferring the light charge accumulated in the PD 102 to the FD 104. It should be noted that the reset switch 107 is turned off prior to the transfer. From time t32 to a time t35, when φTXn becomes low and transfer ends, is a charge accumulation time. After the transfer in the line n ends, a pulse is applied to φSELn and the selection switch 106 is turned on, converting the light charge held in the FD 104 to voltage and outputting it to the output circuit 111. The output circuit 111 is driven by the horizontal scan circuit 110, and the signals temporarily held at the output circuit 111 are output in succession from a time t36. From the start of transfer at time t34 to the end of output at time t37 is T3read, and the time from time t31 to time t33 is T3wait. The process is the same for the remaining lines, with the time from the start of transfer to the end of output being T3read and the time from the start of reset of one line to the start of reset of the next line being T3wait.
A problem with the rolling electronic shutter shown in FIG. 12 is that the charge accumulation timing shifts between the top part of the screen and the bottom part of the screen time by the length of time required to scan the screen. This is because the time T3wait from the start of reset of one line to the start of reset of the next line must be set to a duration that is greater than the time T3read from the start of transfer to the end of output. If T3wait is shorter than T3read, the following problem occurs when attempting to make the charge accumulation time the same for all lines: Specifically, before output of the signals of line n temporarily held in the output circuit 111 ends, the signals of the next line are transferred to the output circuit 111 while the signals of the pixels of line n still remain in the output circuit 111. This not only makes it impossible to output the signals of the pixels of a part of the line n, but the remaining signals of the line n are also added to the signals of line n+1, leading to the wrong signals being output as the signals of the line n+1. In addition, if the signal output cannot be scanned from the output circuit 111 at high speed, then, particularly in the case of a large number of pixels, the shift in the charge accumulation timing (that is, the image sensing timing) from the top of the image to the bottom of the image increases.
Moreover, as is described in Japanese Patent Application Laid-Open No. 2003-17677, there is also a MOS-type image sensing device that performs reset and transfer of charges collectively. The sequence of operations of this sort of operation is shown in FIG. 13. In FIG. 13, all the lines are reset simultaneously from a time t41 to a time t42. In addition, between a time t43 and a time t44 transfer of charges is also performed simultaneously. Hereinafter, this type of electronic shutter is referred to as a collective transfer electronic shutter. With a collective transfer electronic shutter, the charge accumulation time for all lines is from t42 to t44, achieving an electronic shutter with no shift in charge accumulation timing from the top to the bottom of the image.
In addition, in order to carry out reset and transfer of charges at high speed, there is also a MOS-type image sensing device that performs the sequence of operations shown in FIG. 14 designed to increase the speed of reset and transfer of charges. A description is given of this sequence using FIG. 11 and FIG. 14.
First, by applying pulses to φRESn and φTXn from a time t51 to a time t52, the reset switch 107 and the transfer switch 103 of each pixel on the line n are turned on, resetting the PD 102 and FD 104 of each pixel on the line n. From time t52 the charge accumulation that generates a light charge on the PD 102 of each pixel on the line n starts, and at a time t53 the resetting of all the lines up to and including the last line ends. From a time t54 to a time t55 a pulse is applied to φTXn, turning on the transfer switch 103 of each pixel on the line n and transferring the light charge accumulated in PD 102 to the FD 104 of each pixel on the line n. The time from time t52 to time t55 is the charge accumulation time for line n. Transfers for line n+1 and all lines thereafter are carried out in succession from time t55, with transfers for all lines completed at a time t57.
After transfer of the light charge of each pixel on the line n ends, a pulse is applied to φSEL at a time t55, turning switch 106 on. This causes the charge held in the FD 104 to be converted into voltage by a source follower circuit composed of the MOS amplifier 105 and the constant current source 109 and output to the output circuit 111. The signals temporarily held in the output circuit 111 are output in succession to the output line 112 from time t56 by the horizontal scan circuit 110 control. Output of line n+1 is carried out from a time t58, after all the signals of line n have been output from the output circuit 111. The sequence shown in FIG. 14 enables the time from the start of scanning of one line to the start of scanning of the next line to be determined independently of the output time, thus enabling distortion of an image by the rolling electronic shutter to be reduced. This sort of electronic shutter is referred to as a rolling transfer electronic shutter. With the collective transfer electronic shutter, the rolling electronic shutter and the rolling transfer electronic shutter there is no need to use a mechanical shutter, and thus these shutters are optimal for moving image applications.
At the same time, a structure is known that removes noise unique to the pixels from the light signals output from the pixels. As one example thereof, the light signals output from the pixels on one line are temporarily held in capacitors, respectively, the noise signals of the same pixels are temporarily held in other capacitors prior to or after light signal output, and the noise signals are then subtracted from the light signals at each of the pixels. This operation is performed sequentially for all of the lines. Configuring and controlling an apparatus in the foregoing manner enables the noise component to be reduced.
However, because there are slight differences between the capacitors that holds the light signals and the capacitors that holds the noise signals, to further reduce the noise component accurately a structure is known in which clamping circuits are used. With a structure that uses clamping circuits, in the case of a rolling electronic shutter, first, the noise signals of the pixels of the output line are output and made reference levels of the clamping circuits, and at the same time the noise signals are output and held in capacitors. Thereafter, the light signals are output and held in other capacitors after being clamped by the clamping circuit. Therefore, the light signals are clamped by the noise signals, enabling light signals minus noise signals to be obtained from the clamping circuits.
By contrast, in the case of a collective transfer electronic shutter or a rolling transfer electronic shutter, the transfer of the light charge from the PD to the FD is carried out prior to output. As a result, it is not possible to realize a sequence of operations in which the noise signal is output and then the light signal is output after the noise signal is clamped during readout of the light signal. The light charge has already been transferred to the FD when the FD is initially scanned, and therefore the voltage of the FD which is output first becomes the reference level of the clamping circuit. If the noise signal is then output, then the output from the clamping circuit becomes the noise signal minus the light signal, which would be outside the operating range of the amplifier used in the later stage of the clamping circuit.